KABUL, Afghanistan — Abdullah Abdullah, a longtime opponent of President Hamid Karzai and an ardent supporter of the United States, emerged Saturday as the clear front-runner in Afghanistan’s presidential election.

In preliminary results released Saturday, Mr. Abdullah had won 45 percent of the vote, not enough to avoid a runoff with Ashraf Ghani, a former World Bank economist and Karzai adviser, who had won 32 percent. But Afghan government officials say Mr. Abdullah is on the verge of forging alliances with at least two of the runners-up to gain their support, and possibly the presidency, in the next round.

Either of the top two candidates would represent a significant break with the years of deteriorating relations the United States has had with Afghanistan under Mr. Karzai, and a shift toward greater bilateral cooperation. Each candidate has said, for instance, that he will sign a security agreement allowing American forces to remain in the country past 2014, which Mr. Karzai negotiated but refused to sign.

But the United States and its NATO allies were likely to see the apparent advantage for Mr. Abdullah, with his long record of advocating closer relations with the United States and a more militant stance against the Taliban, as encouraging, although they have been careful not to express support for any candidate in the race.

Mr. Abdullah, a northerner of mixed Tajik and Pashtun ethnicity, would also become one of the few northerners to lead a country long dominated by Pashtuns from the south.

The election, the third for president since the NATO-led invasion of 2001, appears to have been the country’s most democratic yet. The turnout was roughly 50 percent higher than that of the last election, the deeply tainted race that Mr. Abdullah lost to Mr. Karzai in 2009. Early indications suggested that it was also far cleaner than the last one, although final rulings on fraud complaints may not come for several weeks.

Mr. Ghani was the leading Pashtun candidate, but the other top two Pashtuns — Zalmay Rassoul, believed to have been Mr. Karzai’s favorite, and Gul Agha Sherzai, a former warlord favored by the C.I.A. and popular in the Taliban’s southern heartland — were expected to throw their support to Mr. Abdullah, according to two senior Afghan government officials.

Their support could give Mr. Abdullah a powerful mandate if he wins the runoff, which will be held no sooner than May 28. In a recent interview, he said he would set a different tone with the United States, ending the often acrimonious criticism from the Afghan president over prisoner releases, civilian casualties and night raids. “This rhetoric has not helped Afghanistan,” he said.

Mr. Karzai, who is stepping down after 12 years in power, has been studiously neutral throughout the campaign and has maintained silence on the issue since the April 5 election. Officials in the presidential palace have said he is deeply worried about Mr. Abdullah’s apparent success.

Ethnic divisions are important here, not least because of the Taliban’s largely Pashtun base. The largest ethnic group in Afghanistan, Pashtuns are believed to represent 42 percent of the population and have taken leading Afghanistan as a birthright ever since Ahmad Shah Durrani created an Afghan-based empire in 1747 that ruled much of present-day Iran, India and Pakistan.

Many moderate Pashtuns worry that a government led by Mr. Abdullah, whose power base has been among Tajiks in northern Afghanistan, would drive more Pashtuns into supporting the insurgents. That made his expected alliances with the Pashtun candidates, Mr. Rassoul and Mr. Sherzai, critical in a second round.

Mr. Ghani could still cut enough deals with some of the other six losing candidates to regain the ground lost to Mr. Abdullah.

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Ashraf Ghani, a former World Bank economist and Karzai adviser, won 32 percent of the vote and will likely face Mr. Abdullah in a runoff.CreditDiego Ibarra Sanchez for The New York Times

Or, as has happened many times in the past, Afghans might vote their ethnicities, with the more numerous Pashtuns all rallying to Mr. Ghani’s side. Mr. Abdullah dismissed that possibility, saying Afghans had “risen beyond” ethnic politics.

Mr. Ghani is closely associated with Mr. Karzai’s government, serving in the past three years as his adviser in charge of transition, the process in which responsibility for security was being gradually transferred from the Americans and NATO to Afghan security forces.

In an interview Saturday, Mr. Ghani cited that work as an indication of the less contentious relationship with the United States he espouses. “Neither you nor any journalist can cite a single incident where any aspect of transition became a public debate or an issue of contention,” he said.

But some voters assign him a measure of guilt by association for Mr. Karzai’s refusal to sign the bilateral security agreement with the United States, a move that has thrown the future of a Western military presence here into doubt and turmoil. The deal had widespread support among Afghans and was endorsed by the grand council of elders, or loya jirga, that Mr. Karzai called to ratify it.

American officials have expressed alarm that the lack of an agreement would force a total American military withdrawal by the end of the year and the potential loss of air bases used for drone strikes in Pakistan. Some American policy makers fear it could also ease the way to a Taliban resurgence and even a regrouping of Al Qaeda, whose presence here the 12-year NATO-led war was intended to eliminate.

While Mr. Ghani has also promised to sign the security agreement if he takes office, he has refrained from criticizing Mr. Karzai on the issue. Mr. Abdullah, on the other hand, has assailed Mr. Karzai for it, saying his refusal to sign had imperiled Afghanistan’s security in the midst of a war.

Mr. Ghani also was deeply involved in the bitter disputes between Mr. Karzai’s government and the Americans over the transfer of Taliban prisoners to Afghan control and their subsequent quick release, which Mr. Abdullah criticized as sending them directly back to the battlefield.

Mr. Abdullah, whose father was a Pashtun from Kandahar, has been more closely identified with the Tajiks in the north, who were central in the fight against the Pashtun-led Taliban, which swept to power in most of Afghanistan in the 1990s, except for small parts of the north under Northern Alliance control.

During the Taliban government, from 1994 to 2001, Mr. Abdullah, an ophthalmologist by training, served as the spokesman for Ahmed Shah Massoud, the northern leader assassinated by Al Qaeda days before the Sept. 11, 2001, attacks. Mr. Abdullah was one of Mr. Massoud’s few confidantes who could speak English, and had a close relationship with the alliance’s C.I.A. handlers at the time.

In the first Karzai government after the American-led invasion toppled the Taliban, Mr. Abdullah served as foreign minister, while Mr. Ghani was the minister of finance. Mr. Abdullah acquired a reputation for suavity, and a penchant for expensive suits and Louis Vuitton slippers. Mr. Ghani was widely admired for his intellectual wattage, but often infuriated other officials who complained he was condescending and prickly.

During the 2014 campaign, they met several times in televised debates. Mr. Abdullah said his whole strategy was to nettle Mr. Ghani into losing his temper; he is famous for tantrums which have alienated many Afghan officials, and which have worried American military leaders who have dealt with him regularly during the transition process.

There was little evidence of that during the campaign, however.

“He was the one who was angry during campaign debates,” Mr. Ghani said. “Isn’t that ironic? If the campaign has shown anything it’s that my alleged reputation is manufactured.”

Mr. Abdullah often boasted during the campaign that he had never left Afghanistan, remaining to fight with the Northern Alliance while the Taliban were in power. Mr. Ghani, on the other hand, was a longtime exile and lived in the United States; his wife is an American citizen of Lebanese descent.

Many Afghans have been resentful of exiles who escaped the hard years of civil war and the Taliban government, only to return to fame and fortune in 2001 with connections in Western countries that gave them advantages in business and government.

Correction:June 3, 2014

An article on April 27 about early results in the Afghan presidential election, which showed Abdullah Abdullah as the clear front-runner, contained a number of errors in some editions. Mr. Abdullah is part Pashtun; it is not the case that he is “non-Pashtun.” His father was Pashtun, not Tajik. And Mr. Abdullah did not in fact have a stepfather, according to Ali Farhad, a spokesman for his campaign. In addition, Afghanistan has had several non-Pashtun leaders in the last three centuries; it is not the case that there were none during that time. And the turnout in this year’s presidential election was about 50 percent higher than in 2009, not “nearly double.” This correction was delayed for research.